The invention relates to a device for measuring the uniformity of a vehicle tire, of the type defined more closely in the pre-characterising portion of claim 1. The running smoothness of a vehicle and the wear of various vehicle components depend on the uniformity on otherwise of the vehicle""s tires. Force fluctuations produced by the tires during driving also influence the running smoothness of the vehicle. To be able to assess the quality of the vehicle tires, tires of the most varied sizes, with and without loading of the tire, are subjected to low and very high speeds.
EP 0 897 107 A2 discloses a device for measuring the uniformity of a vehicle tire, in which a roller can be displaced on a frame in such manner that a vehicle tire fixed and driven in a test rig comes into contact with the said roller. The roller is fixed in a sliding carriage by means of pressure bearings constructed as roller bearings. The vehicle tire is held in a measurement device for measuring reaction forces, the measuring device illustrated in FIG. 5 having at least three measuring transducers formed as two-component force-measuring elements operating on a piezo basis. With this type of force-measuring element, force variations in the radial, lateral and tangential area can be determined. An exact measurement of the static lateral forces cannot be carried out with the device for measuring the uniformity of a vehicle tire disclosed in FIG. 5. The solutions illustrated in FIGS. 1 to 4 are unsuitable for high-speed measurements on account of insufficient rigidity.
The purpose of the present invention is to provide a device for measuring the uniformity of a vehicle tire, with which all the forces acting on the wheel hub can be determined even when the tire is rotating at high speed.
This objective is solved with a device for measuring the uniformity of a vehicle tire, which is of the type described but also embodies the characterizing features of the main claim.
According to the invention a vehicle tire to be tested, or even a complete wheel, is positioned in a holding device which is mounted via differential force sensors. The holding device for the vehicle tire preferably consists of a divided measuring wheel-rim system, which can also be provided with an automatic chuck width adjustment in order to enable rational testing of vehicle tires during production. The differential force sensors are preferably arranged in a multi-component piezo measuring hub which is attached rigidly to the machine frame and which accommodates the holding device for the vehicle tire. A driven roller is mounted to rotate on a sliding carriage arranged on a frame upon which it can be displaced. The roller and the vehicle tire to be tested are positioned horizontally, thanks to which, with a divided measuring wheel-rim system, simple transport to and from the device for measuring the uniformity of a vehicle tire is made possible. The roller can be driven by a drive motor whose speed is adjustable. By moving the sliding carriage with the roller against the vehicle tire to be tested, a defined wheel load can be established. In the radial direction the roller is mounted on radial bearings, preferably hydraulic bearings of a hydrostatic or a hydrodynamic design. Thanks to the use of such bearings, the roller can be displaced in the axial direction. The roller is axially supported exclusively by a de-coupled axial bearing, itself supported in force-measuring cells. The drive motor of the roller can also be de-coupled from the roller by couplings that are xe2x80x98softxe2x80x99 in the axial direction, so~that the lateral forces of the vehicle tire can be picked up by the force-measuring cells virtually without friction and without excessively high measurement peaks. The diameter of the roller is preferably similar to those in tire test rigs operating at a speed of about 60 rpm. This small roller diameter enables the acceleration phase for testing the vehicle tire at high speeds to be kept short. Since once the roller has accelerated, the vehicle tire it is withdrawn from the tire, out-of-balance forces of the freely rotating wheel can also be determined. Thus, all the forces and torques acting on the wheel hub, i.e., all the forces and torques that result from imbalance, rigidity fluctuations, out-of-round defects or mass defects, can be determined. Thanks to the separate measurement of the static lateral force, cone and angle effects too can be measured with high accuracy. The device for measuring the uniformity of a vehicle tire can also be fitted with measuring devices for measuring the geometry of the vehicle tire, such as the concentricity and planarity of its running.